The present invention relates generally to the field of optical mark reading (OMR) and data image-scanning and capture from one or both sides of documents that are transported across a scanning station by a mechanical feeding means. More particularly, the present invention relates to an improved optical light-source, digital reading or detecting head apparatus, and the supporting data processing logic to accurately and consistently calibrate the illumination source for the scanning station and/or the image detector such that the true darkness level of pre-printed or manually entered data may be extracted from each scanned document.
Optical read head systems (hereinafter referred to as ORHS), that capture information printed, stamped, photographed, photocopied, manually entered, or otherwise placed on either one or both sides of a document surface are well known in the prior art. There are numerous applications, in the field of document scanningxe2x80x94both OMR and image-capturexe2x80x94that require a spectral discrimination capability within the ORHS. For example, a given OMR form may be pre-printed in red ink (e.g., the data-entry marking xe2x80x9cbubblexe2x80x9d positions, etc.) and the user permitted to enter the data with any marking instrument but red: for example, a lead-pencil and/or black/blue/green ball-point pens or felt-tip markers. Spectral discrimination permits the user marks to be detected while the red ink is not detected.
It will be appreciated that users desire the flexibility to utilize OMR and image-capture forms that may be pre-printed with a wide variety of colored inks, and offer marking entry with a wide range of marking instruments. Accordingly, a sophisticated ORHS must offer the means to quickly and selectively set the spectral parameters to achieve the desired range of data detection and the desired range of pre-printed ink rejection or non-detection.
Prior art ORHS and associated feeding means typically have been configured for only one detection mode; that is, only pencil marks can be detected, or only red ink is not detected, or all data on the form is detected, such as for the general-purpose image-capture systems widely available on the market today.
Some prior art systems offer the option of changing modes, either by manually exchanging the illuminating source in the ORHS (swapping miniature fluorescent lamps, for example), or changing an optical-path filter to shift the relative spectral response of the light-source and/or detector in the ORHS.
Yet other approaches modify the detect/non-detect parameters in the application software in an effort to achieve the selective discrimination discussed earlier. While these approaches accomplish the minimum goal of accommodating a wider range of document designs and applications than would otherwise be possible with a fixed-spectrum design, this flexibility is not necessarily convenient to utilize in a xe2x80x9creal worldxe2x80x9d environment where the downtime to make the necessary changes is costly, and/or requires the services of a field-engineer or other highly-skilled operator on the user""s staff.
Furthermore, prior systems have suffered from lack of accuracy in the data-detection process when the detection/discrimination mode is changed frequently, unless great care is taken to calibrate the settings of the light-source, detectors, or signal interpretation logic to recognize and process the different signal-contrast levels that inevitably result when such manual-intervention changes are made to the ORHS configuration. While such calibration may be possible, in most systems calibration is complex and/or time consuming.
Also, existing ORHS""s are generally configured to run at a fixed document feeding rate, for example, 3,000 sheets/hour. This fixed feeding rate is often referred to as the maximum pick rate at which sheets can be transported past the ORHS scan-axis, with no regard to throughput degradation due to software-bound latencies, feed jams, or other causes of loss of throughput. The maximum sheets/hour rate is primarily set by the velocity of the sheet as it travels though the feeding mechanismxe2x80x94from the input hopper, through the scan-axis, to the output hopper or shunt stackers.
Prior art scanners run at a fixed velocity rate primarily because changing the rate (not that difficult to accomplish from a mechanism design viewpoint) requires significant adjustment settings to the ORHS. For example, the widely used charge-coupled device (CCD) linear array detectors that form the basis of nearly all image-capture systems detector front ends, require a specific light-source illumination level on the sheet surface for a given sheet velocity.
Image-capture applications, by contrast to OMR, require pixel resolutions ranging from 120 DPI (dots/inch) to 300 DPI, or even higher, depending upon the specific application and quality level of the detected and captured image. An OMR application can take advantage of the higher DPI resolution inherently required in image-capture applications by further improving the detection of weak or mis-registered OMR marks, but the OMR mode does not necessarily require access to all pixels available in the image-capture mode.
It is generally desirable in the image-capture mode to have the DPI equal in both the X and Y directions on the document (e.g., the horizontal xe2x80x9csweepxe2x80x9d direction, and the vertical document feed-direction, respectively). Therefore, the higher the resolution, the slower the sheet must travel under the x-direction scan-axis for a given CCD clock rate.
A consequence of this inherent difference in DPI detection resolution requirements for OMR applications versus general purpose image-capture applications is that prior art scanning systems that attempt to offer optimized operation for both OMR and general-purpose image-capture modes fall short in both modes. A traditional OMR design for the ORHS would suffer from lack of DPI resolution in an image-capture mode, since it is optimized to meet the high throughput demands of OMR applications; on the other hand, the higher DPI resolution requirements of an image-capture mode severely limit the sheets/hour throughput capability when reprogrammed to extract OMR data only.
Therefore, customers who have both extensive OMR and image-capture applications are often forced to acquire at least two different prior-art scanning systems to meet their throughput demands, since the xe2x80x9ccompromisedxe2x80x9d designs of prior-art multiple-mode or mixed-mode systems fail to serve either application mode sufficiently well.
The present invention offers an ORHS solution for incorporation into any scanning document feeding means that can automatically and quickly switch back and forth between OMR and image-capture modes (e.g, user programmable), and scan the respective documents at different sheet velocities under the scan-axis to optimize performance in either mode.
The present invention also offers an ORHS that enables a user to automatically select a desired spectral range for detection of the desired marks or images while rejecting certain pre-printed inks on the forms.
The present invention further provides an ORHS that automatically adjusts, stabilizes, and monitors the output of the ORHS such that accurate data extraction is possible over prolonged periods of time without operator attention or intervention.
Additional advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following.
The present invention involves a method for calibrating an optical reading head system""s pixel output, where the pixel output comprises a discrete value for each of a plurality of pixels and the system comprises (1) a scanning station with an associated plurality of light sources, and (2) a detector having an array of photosensitive sites that sense light reflected from the scanning station during a detector exposure period. The sensed light is utilized to generate a pixel output profile. The method comprises: supplying at the scanning station calibration media having a calibration region; defining for the system with respect to the calibration region a target pixel output profile; illuminating the calibration region with the light sources to generate a pixel output profile of the calibration region; and adjusting the amount of light energy available to the photosensitive sites during a detector exposure period until the pixel output profile of the calibration region is acceptably within range of the target pixel output profile.